Synchronizing a telecommunications connection in a mobile communications system

ABSTRACT

A method of synchronizing a telecommunications connection in a mobile communications system having at least one base station, and having a first and at least a second mobile station communicating on a direct mode channel, the first mobile station operating on the direct mode channel transmits a registration message to the base station of the mobile communications system on a system channel of the mobile communications system, the registration message indicating a wish of the first mobile station to operate in dual watch mode on the control channel of the mobile communications system. In response to the registration message, a specific control channel of the base station and the timing information related to the control channel are communicated to the first mobile station, the first mobile station switches to dual watch mode in order to communicate either on the direct mode channel or on the allocated control channel during the time indicated by the timing information.

This application is the national phase of international applicationPCT/FI96/00436, filed Aug. 13, 1996 which designated the U.S.

FIELD OF THE INVENTION

The invention relates to a method for synchronizing a telecommunicationsconnection in a mobile communications system comprising at least onebase station communicating on system channels of the mobilecommunications system, the channels consisting of control channels andtraffic channels; a first and at least a second mobile stationcommunicating on a direct mode channel divided into frames consisting oftime-slots.

BACKGROUND OF THE INVENTION

The invention relates to the field of mobile telephone systems. Asubscriber of a mobile telephone system, that is, a subscriber station,e.g. a mobile telephone or a mobile station may be connected to a mobilenetwork or system, whereby it is connected to the mobile network viasystem channels maintained by the base stations of the mobile network,the system channels consisting of control channels and traffic channels.

In addition to the system channels located in mobile telephone systems,so-called direct mode channels may be used in association with themobile system, i.e. communication takes place in a direct mode.Subscriber stations communicating in the direct mode do not communicatedirectly with the mobile network or the base stations related to it.Direct mode channels are frequencies on which mobile telephones or othermeans of communication can communicate without the system directly witheach other, or via repeater stations, either with the base stations ofthe system or with other mobile stations.

Direct mode channels are typically used in situations in which e.g., agroup of hand-held mobile phones which communicate with each other areso remote from the base stations that the use of system channels is notpossible.

Another important use of direct mode channels is increasing the capacitywhen the traffic in the system rapidly increases, e.g. due to anincident occuring in some part of the service area of the system, e.g.in some spot-like part.

A direct mode channel is a channel that is typically not used by thesystem. It may be e.g., a channel whose channel spacing is the same asthat in the mobile communications system, e.g. 12.5 kHz or 25 kHz. Ondirect mode channels, a mode of simplex operation is used. From thegroup of the mobile stations operating on the direct mode channel, thetransmitting party has tuned his or her transmitter onto the channel andis transmitting speech or data information. The other mobile stationsthat have been activated onto the direct mode channel have tuned theirreceivers onto the same frequency so that they will directly hear thetransmission.

On a direct mode channel, communication may take place by means ofanalog modulation or digital modulation. A mobile station transmittingon the channel may also use signalling information, such as informationon access rights and priorities, or the group communicating on thechannel, or data communication. On a direct mode channel, encryption maybe carried out, or plain speech can be transmitted.

Subscriber stations operating in the direct mode communicate with othersubscriber stations on a direct mode channel without having a directconnection to the base stations of the mobile network. Subscriberstations operating in the direct mode, however, may communicate with themobile network via repeater stations. A repeater station is a unit inwhich there are two transceivers connected to each other. A repeaterstation typically comprises two transceivers. The repeater stationforwards information messages transmitted on the direct mode channel tothe desired network elements, such as the base stations of the radionetwork, which will forward the information messages to the exchanges ofthe radio network. Information messages forwarded by the repeaterstation may be e.g., speech, or data or signalling messages. When therepeater station is being used, e.g. a semiduplex mode of operation maybe used on the direct mode channel.

A problem related to a prior art solution is how to make the mobilestation operate upon request in accordance with the dual watch mode,that is, to communicate "simultaneously" on the direct mode channel andthe system channels of the mobile communications system. A mobilestation operating in the dual watch mode on the direct mode channel mayreceive transmissions of the system channels of the mobilecommunications system between time-slots received on the direct modechannel. A dual watch function of this kind has not been implementedbefore between a mobile station operating on the direct mode channel,and a mobile communications system. This kind of a situation is attendedby the drawback that the frame synchronization of the direct modechannel will be changed with respect to that of the mobilecommunications system, in which case it is difficult or even impossiblefor the mobile station to make use of the dual watch mode.

Another problem related to prior art solutions is the fact that themobile communications system does not know whether the mobile station isin the dual watch mode or not, in which case it is difficult for thesystem to co-operate with the mobile station possibly operating in thedual watch mode.

Yet another problem related to prior art solutions is the fact that themobile communications system does not know the identifier of the directmode channel on which the mobile station in the dual watch mode isoperating.

SUMMARY OF THE INVENTION

The object of this invention is to implement a method and a mobilecommunications system by means of which the problems related to theprior art may be solved.

The object of the invention is to enable the use of the dual watch modein a mobile station that is operating on a direct mode channel and alsocommunicating in the dual watch mode on the system channels of themobile communications system.

This novel method for synchronizing a telecommunications connection willbe achieved with a method in accordance with the invention, which ischaracterized by the steps of: a first mobile station operating on thedirect mode channel transmitting a registration message to the basestation of the mobile communications system on the system channel of themobile communications system, by means of which registration message thefirst mobile station communicates its wish to operate in the dual watchmode on the control channel of the mobile communications system; inresponse to the registration message, communicating to the first mobilestation a specific control channel of the base station and the timinginformation related to the control channel; the first mobile stationswitching to the dual watch mode for communicating either on the directmode channel or the allocated control channel during the time indicatedby the timing information.

The invention further relates to a mobile communications systemcomprising: at least one base station communicating on the systemchannels of the mobile communications system, a first and at least asecond mobile station operating on a direct mode channel divided intoframes consisting of time-slots.

The mobile communications system of the invention comprisescommunicating means for communicating the specific control channel ofthe desired base station and the timing information related to thecontrol channel to the first mobile station operating on the direct modechannel.

The invention further relates to a mobile station operating on a directmode channel divided into frames consisting of time-slots, the mobilestation comprising: a memory means, a transceiver which is arranged totransmit and receive transmissions of the mobile station, a controllerfor controlling the operation of the mobile station.

The mobile station of the invention comprises transmission means fortransmitting the timing information to one or more other mobile stationson the direct mode channel, the timing information containing theidentifier of the mobile communications system used and its controlchannel, as well as the timing information related to said controlchannel.

The mobile station of the invention comprises synchronizing means forshifting the start time of a frame or a time-slot of a direct modechannel transmission with respect to the start time of a frame or atime-slot in accordance with the synchronization of the control channelof the mobile communications system in response to the synchronizationinformation transmitted by the second mobile station, or a base stationor a repeater station of the mobile communications system.

The invention is based on the idea that the timing of the transmissionof the mobile stations operating on the direct mode channels in the dualwatch mode is synchronized with the transmission of the control channelsof the base station of the mobile communications system. The mobilestations operating on the direct mode channel are synchronized in such amanner that when a mobile station operating on the direct mode channelwishes to operate in the dual watch mode, it will report to a basestation of the system and communicate, when necessary, its wish tooperate in the dual watch mode on said direct mode channel and on thecontrol channel of the mobile communications system. In addition, themobile station may provide the mobile communications system with theidentifier of the direct mode channel on which it is operating.Thereafter, the mobile communication system will accept the mobilestation to the system and assign to it an appropriate control channelfrom the desired base station for the dual watch mode, and indicate thesynchronization of the channel. Thereafter, the mobile station willshift to the direct mode channel and adapt the time-slot and framesynchronization of its own transmission, and, if need be, the time-slotand frame synchronization of the other mobile stations operating on thedirect mode channel in order that it will correspond to thesynchronization of the control channel of the mobile communicationssystem. In other words, the transmission of the mobile station and thedual watch operation are timed in order that the transmission time-slotof the mobile stations will allow the mobile stations operating on thedirect mode channel to listen to the control channel between thetransmission time-slots of the direct mode channel.

The above-mentioned adaptation is carried out by means of either of theembodiments disclosed in the following:

In accordance with a first embodiment, the adaptation is carried out ina situation in which there is no ongoing traffic on the direct modechannel in question, that is, the direct mode channel that is intendedto be used is free. Thus, the mobile station will transmit asynchronization identifier on the direct mode channel to other mobilestations operating on the direct mode channel. The synchronizationidentifier contains e.g., at least a part or all of the followinginformation: the identifier of the transmitting mobile station, theidentifiers of the mobile communications system and of the controlchannel used, the information on the carrier wave of the controlchannel, as well as the information on timing, that is, on thehyperframe, the multiframe and the time-slot. The mobile station maytransmit the synchronization identifier several times at desiredintervals. The other mobile stations operating on the direct modechannel will receive this synchronization identifier and synchronizetheir own transmission by means of it so that the transmission of themobile stations will be arranged in the way disclosed above.

In accordance with a second embodiment, the adaptation is carried out ina situation where a mobile station is already transmitting on the directmode channel, that is, where it is endeavored to alter thesynchronization during a call. In accordance with the second embodiment,the mobile station will alter the synchronization of its transmissiontaking place on the direct mode channel only after it has detected thatsome other mobile station is transmitting on the direct mode channel.Altering the synchronization may be carried out as follows in accordancewith the different variations of this embodiment:

1. The frame synchronization will be altered during the transmissionturn of the other mobile station by interrupting the transmission turnof the other mobile station and re-synchronizing it in the desiredmanner.

2. A second, probably the most practical way is to alter thesynchronization during a call, but between the transmission turns. Inother words, the synchronization will be altered after the transmissionturn of the other mobile station has ended, by sending a synchronizationmessage.

3. The third way is to carry out the synchronization at the beginning ofthe actual transmission turn of the mobile station.

This embodiment has the advantage that the mobile station that iscarrying out/altering the synchronization of the direct mode channeldoes not need to transmit a synchronization identifier unless it is surethat there is ongoing traffic on the direct mode channel in question,e.g. by listening to the direct mode channel in question.

The invention is further based on the idea that the frame structures ofthe system channel and the direct mode channel must be in synchronismwith each other for implementing the measures related to the dual watchmode and simplifying the maintenance of the frame synchronization. It isanother object of this invention that, upon registering to the system, amobile station operating in the dual watch mode reports that it isoperating in the dual watch mode and provides the identifier of thedirect mode channel on which the mobile station is operating. It is alsopossible for the mobile communications system to use a common address(identifier) it has determined for the mobile stations, for managing thetraffic taking place on the direct mode channel.

It is optional for the method of the invention that the mobile stationcommunicates to the mobile communications system that it is operating inthe dual watch mode. It is also optional for the mobile communicationssystem to treat the mobile station in the dual watch mode differentlyfrom the other mobile stations. On the other hand, the system maycommand all the dual watch mobile stations onto a specific controlchannel for avoiding the situation in which two mobile stations areconnected to the same base station, but listening to separate controlchannels, i.e. attempting a dual watch synchronization to separatetime-slots.

An advantage of the invention in accordance with this application isthat it solves the problems related to the prior art solutions.

Another advantage caused by implementing the frame synchronization inaccordance with the invention is the fact that a mobile stationoperating in the dual watch mode only requires one system formaintaining the synchronization.

Another advantage of the invention is the fact that planning the dualwatch mode will be facilitated since the synchronization between themobile stations and the system may be assumed to be stable on account ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be disclosed in closer detail withreference to the attached drawings, in which

FIG. 1 shows a block diagram of a mobile communications system of theinvention in which the method of the invention may be applied,

FIG. 2 shows the frame structure of the TETRA mobile communicationssystem,

FIG. 3 shows the registration of a first mobile station DMRU1 to themobile communications system A and transmission of a dual watchsynchronization identifier on a direct mode channel f1,

FIG. 4 shows the registration of a mobile station and starting theoperation on the direct mode channel,

FIG. 5 illustrates implementing time-slot synchronization in accordancewith the invention for achieving a dual watch mode between the directmode channel and the control channel of the mobile communicationssystem,

FIG. 6 illustrates implementing frame synchronization in accordance withthe invention for achieving a dual watch mode between the direct modechannel and the control channel of the mobile communications system,

FIG. 7 shows a block diagram of the mobile station of the inventionoperating on the direct mode channel,

FIG. 8 shows a flow chart of the synchronization of the system channeland the direct mode channel frame structures according to the invention,and

FIG. 9 shows a flow chart of the synchronization of the system channeland direct mode channel frame structures in accordance with theinvention.

DETAILED OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a block diagram of a mobile communications system of theinvention in which, e.g., the method of the invention may be applied.The mobile communications system may also comprise mobile stations TMRUoperating on the system channels TMCCH, TMTCH, a direct mode channelDMCH, system channels TMCCH and TMTCH, base stations BS and a systemSwMI. The system SwMI may comprise, e.g., an exchange of the mobilecommunications system, transmission equipments, and an operations andmaintenance center. The invention is particularly related to mobilestations DMRU operating on the direct mode channel DMCH, and especiallythe operation of those mobile stations DMRU in the dual watch mode insynchronism with the control channels of the mobile communicationssystem. FIG. 1 also shows a mobile station DMRU operating in the dualwatch mode.

The invention relates to a mobile communications system (FIG. 1)comprising: at least one base station BS communicating with on thesystem channels of the mobile communications system, and a first and atleast a second mobile station DMRU; 400 (FIG. 7) operating on a directmode channel divided into frames consisting of time-slots.

The mobile communications system of the invention is characterized bycomprising: communicating means 100 for communicating a desired controlchannel of a desired base station and timing information related to saidcontrol channel to the first mobile station DMRU; 400 (FIG. 7) operatingon the direct mode channel.

FIG. 2 shows the frame structure of the TETRA mobile communicationssystem. On system channels, the frame structure of the uplink frequency,that is, the direction from the mobile station to the base station(RU→BS), is delayed by two time-slots compared with the frame structureof the downlink frequency, that is, the direction from the base stationto the mobile station (BS→RU) in accordance with the TETRA protocol. Thedirect mode channel is also divided into frames and time-slotsphysically in the same way. There is, however, only one frequency, whichis used for communication in both directions.

FIG. 3 shows a situation in which a mobile station DMRU1 wishes tooperate in the dual watch mode between a base station C1 of the system Aand a direct mode channel f1. In FIG. 3, mobile station DMRU1 reports,i.e. registers to the system via the base station C1. In associationwith the registration, DMRU1 also reports it is operating in the dualwatch mode and possibly the identifier of the direct mode channel it isusing. Reporting the identifier of the direct mode channel enables thesystem to shift the mobile stations operating on the same direct modechannel onto the same control channel in accordance with the dual watchmode. In accordance with another approach, the system will direct themobile stations operating in the dual watch mode to the same controlchannel every time only on the basis of the dual watch request made bythe mobile stations, regardless of which direct mode channel is inquestion. The system A will accept mobile station DMRU1 to the systemand assign to it a control channel k1 (MCCH, SCCH) of the base stationC1.

FIG. 4 shows a situation in which the first mobile station DMRU1 shiftsonto the direct mode channel f1. If need be, DMRU1 attempts to adapt thetime-slot synchronization of the direct mode channel f1 to correspond tothe control channel k1 of the entire mobile communications system.Correspondence herein refers to the fact that the transmission time-slotof the direct mode channel f1 will be arranged in such a way that thecontrol channel of the system can be listened to simultaneously. FIG. 5,which will be explained below, illustrates the synchronization betweenthe direct mode channel and the channels of the mobile communicationssystem. In addition, there must be a frame synchronization between thedirect mode channel and the base station of the system.

The adaptation mentioned above may be carried out using two approaches:

In accordance with the first approach, in a case where the direct modechannel is available, that is, there is no ongoing traffic on the directmode channel, mobile station DMRU1 will transmit a synchronizationidentifier that contains the identifier of mobile station DMRU1, theidentifier A of the system, the identifier of the base station C1, aswell as the information related to the control channel k1 (carrier wave,hyperframe, multiframe, time-slot). This synchronization information isthen repeated at appropriate intervals. Once the traffic has started,the transmitting mobile station will indicate that the direct modechannel is in the dual watch mode.

In the second approach, mobile station DMRU1 will alter thesynchronization only when someone is transmitting on the direct modechannel. This approach has the advantage that mobile station DMRU1 doesnot need to transmit the synchronization identifier on the direct modechannel at regular intervals.

A second mobile station DMRU2 to be activated on the direct mode channeland wishing to operate in the dual watch mode will report to the systemA of the same base station C1 as indicated in the synchronizationtransmission of the direct mode channel. The system will command thesecond mobile station DMRU2 onto the same control channel as the one onwhich the first mobile station DMRU1 is, by making use of theinformation related to the first mobile station DMRU1 (dual watch mode,direct mode channel identifier, control channel) and transmitted in thesynchronization identifier mentioned above. Reporting the identifier ofthe direct mode channel to the system enables the fact that the systemmay treat the dual watch mobile stations operating on the direct modechannel as one group.

FIG. 5 illustrates implementing time-slot synchronization in accordancewith the invention for achieving a dual watch function between thedirect mode channel and the control channel of the mobile communicationssystem. FIG. 5 shows the first time-slots of the control channels andthe first and the third time-slots of the direct mode channel with blackcolor for comparing their co-incidence with respect to each other. FIG.5 shows an illustration of frame synchronization in a situation in whichthe transmission of a mobile station or mobile stations transmitting ona direct mode channel is synchronized with the downlink synchronizationof the control channel of the system so that the mobile stations will beable to operate in accordance with the dual watch mode, that is, theywill be able to transmit and receive both on the direct mode channelsand on the channels of the mobile communications system, e.g. receivetransmissions in the downlink direction of the control channel, as shownin FIG. 5. The arrows shown in FIG. 5 illustrate how a transmission turnshifts in accordance with the dual watch mode from the downlinkfrequency of the control channel to the mobile stations operating on thedirect mode channel, and vice versa.

FIG. 6 shows an illustration of frame synchronization, illustrating theimplementation of frame synchronization for achieving a dual watch modebetween the direct mode channel and the control channel of the mobilecommunications system. In the figure, the first time-slots of the systemchannel and the first and the third time-slots of the direct modechannel are marked with black for comparing their co-incideince withrespect to each other. This is illustrated with the arrows shown in FIG.6. The sequence numbers related to the frames of the control channel ofthe system and to the frames of the direct mode channel are also markedin FIG. 6. By means of the arrows shown in the figure, it is possible tomake a comparison and detect that in accordance with the invention, theframe synchronization of the direct mode channel has been so arrangedwith the frame synchronization of the control channel of the systemthat, e.g., frame number 12 of the direct mode channel corresponds tothe frame number 12 of the control channel.

FIG. 7 shows a block diagram of the mobile station 400 of the invention,operating on the direct mode channel, that is, a mobile telephone,mobile station or subscriber station used by a subscriber. The functionof a transceiver station (TX/RX) 401 is to adapt to the radio channelused at a current moment. The transceiver 401 is connected to an antenna402, which communicates with the radio path RP. Radio frequencies thatare normally used are within the range 60-1000 MHz (VHF and UHFfrequency bands), although other frequencies may also be used. On theradio path RP, analog modulation may be employed, whereby the modulationmethod is usually phase modulation. Other modulation techniques may alsobe used. For transmitting signalling, e.g. Fast Frequency Shift KeyingModulation (FFSK) may be employed. Transmission on the radio path mayalso be digital.

A user interface 405 comprises electroacoustic transducer means,typically a loudspeaker 406 and a microphone 407, and possibly buttonsrelated to beginning, ending and dialling a call. As in a trunkingsystem and especially on a direct mode channel a call on the radio pathRP advantageously takes place in turns, the user equipment in most caseshas a push-to-talk button that must be pressed down during the speechturn. The push-to-talk button is not shown in FIG. 7.

The task of a controller 403 is to control the operation of the mobilestation. The controller 403 is connected to the user interface 405, fromwhich it receives impulses associated with call initiation andtermination. The controller 403 may also provide the user with sound orvisual signs related to the operation of the mobile station and/or themobile communications system via the user interface 405.

The controller 403 is connected to a transceiver TX/RX401. The channelused by the transceiver has been determined by the controller 403, thatis, the transceiver 401 is tuned onto the channel, or the radiofrequency determined by the controller 403, and to an appropriatetime-slot. The mobile station of the invention is able to tune onto thedirect mode channel and onto the uplink and downlink channels andfrequencies of the system channels. The transceiver 401 is also switchedon under the control of the controller 403. The controller 403 receivesand transmits signalling messages via the transceiver 401. The mobilestation 400 of the invention communicating on the direct mode channelmay be used, e.g., in a mobile communications system comprising a mobilenetwork comprising at least one base station and subscriber stations,and possibly one or more repeater stations for forwarding trafficbetween at least one base station and the subscriber stationscommunicating on the direct mode channel. In such a case, the mobilestation communicating on the direct mode channel comprises a memorymeans 411, a transceiver 401 and a controller 403 for controlling theoperation of the mobile station.

The mobile station 400 DMRU; 400, (FIG. 7) of the invention, operatingon the direct mode channel divided into frames consisting of time-slotscomprises: memory means 411, a transceiver 401 arranged to receive andtransmit transmissions of the mobile station, and a controller 403 forcontrolling the operation of the mobile station. The mobile station 400according to the invention comprises: transmission means 409 fortransmitting, to one or more second mobile stations, synchronizationinformation containing the identifier of the mobile communicationssystem used and the identifier of its control channel, as well as thetiming information related to said control channel.

The mobile station of the invention DMRU; 400; (FIG. 7) furthercomprises synchronizing means 410 for shifting the start time of a frameor a time-slot of a direct mode channel transmission with respect to thestart time of a frame or a time-slot in accordance with thesynchronization of the control channel of the mobile communicationssystem in response to the synchronization information transmitted by asecond mobile station, or a base station BS or a repeater station RS ofthe mobile communications system. It must be noted that thesynchronizing means may be placed in such a mobile station that alsocomprises the transmission means 409 disclosed above, or in aconventional mobile station that is operable on the direct mode channel.

The mobile station DMRU; 400 (FIG. 7) according to the invention asdisclosed above is arranged to store in its memory 411 a receivedsynchronization information transmitted by a second mobile station DMRU,a base station BS or a repeater station RS of the mobile communicationssystem, the synchronization information indicating the start time of aframe or a time-slot in accordance with the adapted synchronization withrespect to the start time of a TDMA frame or a time-slot.

FIG. 8 shows a flow chart of the synchronization of the system channeland the direct mode channel frame structures in accordance with theinvention. The operation of the method of the invention in the dualwatch mode will start when the mobile station registers 801 bytransmitting its own registration message to the base station of themobile communications system on the control channel. The mobilecommunications system will receive 802 the registration of the dualwatch mobile station and check 803 whether said mobile station isallowed to register to the mobile communications system. In case thedatabase of the mobile communications system contains such aninformation that the mobile station is not allowed 804 to register tothe system, the mobile station will be given a negative answer 805. If,instead, the mobile station is allowed 806 to register to the mobilecommunications system, it will be checked 807 whether the mobile stationis allowed to be in the dual watch mode. In case the dual watch mode isnot allowed 808, the mobile station will not be allowed to startoperating in the dual watch mode. If, instead, the mobile station isallowed 809 to operate in the dual watch mode, it will first be foundout whether mobile stations have already been activated for the directmode channel in question. If this is the case 811, then the controlchannel and the direct mode channel specific address previouslyallocated for the mobile station will be retrieved. Once these measuresfor registrating the mobile station to the network have been carriedout, a positive answer regarding the registration to the network will betransmitted to the mobile station. If, again, no mobile stations havepreviously been activated 812 for the direct mode channel in question, adirect mode channel specific address will be allocated to the mobilestation, and a direct mode channel-specific "stable" control channelwill be allocated to the mobile stations operating on the direct modechannel. Again, after these steps for registering to the network havebeen taken by the mobile station, the mobile station will be transmitteda positive answer regarding the registration to the network, as above.

FIG. 9 shows a flow chart of the synchronization of the system channeland direct mode channel frame structures in accordance with theinvention in a situation in which an attempt is made 901 for activatingthe dual watch mode in a mobile station operating on the direct modechannel. Firstly, it will be checked 902 whether the dual watch mode hasbeen activated on the direct mode channel. If this is the case 903, thesystem in question and the base station related to the system will befound out 904 on the basis of a dual watch message received on thedirect mode channel. Thereafter, the mobile station will report 905 tothe mobile communications system using the base station indicated in thedual watch message, if possible. Thereafter the mobile station will wait906 for the answer from the system, and in case the answer is negative908, the mobile station will not make any re-attempts for activating thedual watch mode with the system. If, instead, the answer is positive909, it will be checked 910 whether the dual watch mode is allowed withthe mobile communications system in question. In case the dual watchmode is not allowed 911, the mobile station will not be able to startoperating in the dual watch mode on said direct mode channel. If,instead, the dual watch mode is allowed 912, the mobile station willstart 913 the dual watch operation on the direct mode channel. Themobile station will thus receive 914 messages from the mobilecommunications system both with its own identifier and with the directmode channel specific identifier in accordance with the dual watch mode.The mobile station will thus wait and monitor 915 whether it isreceiving on the direct mode channel a dual watch message, that is, thesynchronization identifier in accordance with the invention. If the dualwatch message, i.e. the synchronization identifier according to theinvention is received 916 on the direct mode channel, the procedure willreturn to step 914, whereby the mobile station will again receivemessages from the mobile communications system both with its ownidentifier and with the direct mode channel specific identifier inaccordance with the dual watch operation. If the message in question isnot 917 a dual watch message, the procedure will shift to step 918, inwhich the mobile station will transmit synchronization identifiers, i.e.synchronization messages to the direct mode channel at desiredintervals. In the synchronization messages, the mobile station willcommunicate to the other mobile stations operating on the direct modechannel which mobile communications system and base station it is using.Thereafter, in step 919 the mobile station will further wait for anappropriate (new) transmission time for the synchronization message andreceive from the mobile communications system messages (calls) inaccordance with the dual watch mode provided both with its ownidentifier and with a direct mode channel specific identifier.

If, instead, it is detected in step 902 that the dual watch mode is notactivated 921 on the direct mode channel in question, the mobile stationwill report 922 to the mobile communications system using the standard"cell ranking" procedure. The mobile station thus also communicates tothe system its wish to operate in the dual watch mode, and theidentifier of the direct mode channel it is using. The mobile stationwill then wait 923 for the answer from the system and check 924 thequality of the answer. In case the answer is negative 925, the mobilestation will make no attempt to activate the dual watch mode between thedirect mode channel and the mobile communications system. If, instead,the answer is positive 926, it will be checked in step 927 whether thedual watch mode is allowed on the direct mode channel with said mobilecommunications system. If this is not the case 928, the mobile stationwill not attempt to activate the dual watch mode. If, instead, the dualwatch mode is allowed 929, the mobile station will start to operate inthe dual watch mode 930, whereafter the procedure will shift to steps918, 919 and 920 described above.

The drawings and the explanation associated therewith are only intendedto illustrate the idea behind the invention. In details, the method andthe mobile communications system of the invention may vary within thescope of the attached claims. Although the invention has been explainedmainly in connection with trunked mobile communications systems, it isalso possible to apply the invention in other kinds of mobilecommunications systems.

We claim:
 1. A method for synchronizing a telecommunications connectionin a mobile communications system having at least one base stationcommunicating on system channels of the mobile communications system,these system channels including both at least one control channel and atleast one traffic channel, and having a first mobile station arrangedfor communicating on a direct mode channel divided into framesconsisting of time-slots, comprising the steps of:transmitting from thefirst mobile station a registration message to a said base station ofthe mobile communications system on a system channel of the mobilecommunications system, by which registration message the first mobilestation communicates a wish of the first mobile station to operate indual watch mode on a control channel of the mobile communicationssystem; communicating by said base station to the first mobile station,in response to said registration message, identification of a specificcontrol channel of the base station allocated for use by the firstmobile station for operating in dual watch mode, and timing informationrelated to synchronization of said specific control channel; and thefirst mobile station, in response to said communicating, switching todual watch mode for communicating alternatively on said direct modechannel and said specific control channel during times indicated by saidtiming information.
 2. A method as claimed in claim 1, wherein:saidtransmitting includes communicating from the first mobile station tosaid base station, in said registration message, information as to thedirect mode channel on which said first mobile station wishes to operatein dual watch mode.
 3. A method as claimed in claim 1, furthercomprising:transmitting from the first mobile station synchronizationinformation to one or more second mobile stations on said direct modechannel, said synchronization information containing timing informationrelated to said specific control channel; and shifting the start time ofa frame of a time-slot of a direct mode channel transmission of at leastone second mobile station with respect to the start time of a frame or atime-slot in accordance with the synchronization of said specificcontrol channel of said mobile communications system, for achieving anadapted synchronization, which is adapted in accordance with saidsynchronization information transmitted by said first mobile station. 4.A method as claimed in claim 3, wherein:said synchronization informationcontains an identifier for said mobile communications system.
 5. Amethod as claimed in claim 4, wherein:said transmitting ofsynchronization information includes transmitting from said first mobilestation a transmission of said first mobile station in accordance withsaid adapted synchronization so that the start time of each frameaccording to the adapted synchronization to be transmitted on saiddirect mode channel is shifted with respect to the synchronization ofsaid specific control channel such that a first time-slot according tothe adapted synchronization coincides with a first uplink time-slot ofsaid specific control channel.
 6. A method as claimed in claim 5,wherein: said transmitting of synchronization information includestransmitting from said first mobile station in accordance with saidadapted synchronization such that the start time of each frame accordingto the adapted synchronization to be transmitted on said direct modechannel is shifted with respect to the synchronization of said specificcontrol channel so that each frame according to the adaptedsynchronization of said direct mode channel coincides with a respectivecorresponding uplink frame of said specific control channel of themobile communications system.
 7. A method as claimed in claim 4,wherein:said transmitting of synchronization information includestransmitting from said first mobile station in accordance with saidadapted synchronization such that the start time of each frame accordingto the adapted synchronization to be transmitted on said direct modechannel is shifted with respect to the synchronization of said specificcontrol channel so that each frame according to the adaptedsynchronization of said direct mode channel coincides with a respectivecorresponding uplink frame of said specific control channel of themobile communications system.
 8. A method as claimed in claim 13,wherein:said transmitting of synchronization information includestransmitting from said first mobile station a transmission of said firstmobile station in accordance with said adapted synchronization in such away that frames in accordance with the adapted synchronization areshifted in order to each start a respective time-slot designated for therespective frame according to said synchronization informationcontaining timing information related to said specific control channelof the mobile communications system.
 9. A method as claimed in claim 8,wherein:said transmitting of synchronization information includestransmitting from said first mobile station a transmission of said firstmobile station in accordance with said adapted synchronization so thatthe start time of each frame according to the adapted synchronization tobe transmitted on said direct mode channel is shifted with respect tothe synchronization of said specific control channel such that a firsttime-slot according to the adapted synchronization coincides with afirst uplink time-slot of said specific control channel.
 10. A method asclaimed in claim 8, wherein:said transmitting of synchronizationinformation includes transmitting from said first mobile station inaccordance with said adapted synchronization such that the start time ofeach frame according to the adapted synchronization to be transmitted onsaid direct mode channel is shifted with respect to the synchronizationof said specific control channel so that each frame according to theadapted synchronization of said direct mode channel coincides with arespective corresponding uplink frame of said specific control channelof the mobile communications system.
 11. A mobile communications systemcomprising:at least one base station arranged for communicating onsystem channels of the mobile communications system, including on atleast one control channel and on at least one traffic channel; a firstmobile station arranged for communicating on a direct mode channeldivided into frames consisting of time-slots; and communicating meansfor communicating on a specific said control channel of a said basestation timing information related to said specific control channel, tosaid first mobile station, wherein:said first mobile station is, inresponse to receiving from said communicating means an identifieridentifying said specific control channel and said timing information,arranged to switch to dual watch mode for communicating alternatively onsaid direct mode channel and said specific control channel during a timeindicated by said timing information.
 12. A mobile station arranged forcommunicating on a direct mode channel divided into frames consisting oftime-slots, comprising:memory means; a transceiver arranged to receiveand transmit transmissions of said mobile station; a controller forcontrolling operation of said mobile station; and said transceiverincluding transmitter arranged for transmitting, to one or more secondmobile stations on said direct mode channel, synchronization informationcontaining an identifier of a mobile communications system used and anidentifier of a specific control channel allocated to said mobilestation, as well as timing information related to said control channel,wherein: identifier of said specific control channel and said timinginformation are arranged to cause said one or more second mobilestations to switch to dual watch mode for communicating alternatively onsaid direct mode channel and specific control channel during a timeindicated by said timing information.
 13. A mobile station arranged forcommunicating on a direct mode channel divided into frames consisting oftime-slots comprising:memory means: a transceiver arranged to receiveand transmit transmissions of said mobile station; a controller forcontrolling operation of said mobile station; and said transceiverincluding transmitter arranged for transmitting, to one or more secondmobile stations on said direct mode channel, synchronization informationcontaining an identifier of a mobile communications system used and anidentifier of a specific control channel allocated to said mobilestation, as well as timing information related to said control channel,a synchronization for shifting the start time of a frame or a time-slotof a transmission on said direct mode channel with respect to the starttime of a frame or a time-slot in accordance with the synchronization ofsaid specific control channel of said mobile communications system inresponse to the synchronization information transmitted by a secondmobile station, or a base station or a repeater station of the mobilecommunications system.
 14. A mobile station as claimed in claim 13,wherein:said memory means is arranged to store therein saidsynchronization information transmitted by said second mobile station,said base station or said repeater station of the mobile communicationssystem and received by said transceiver, said synchronizationinformation indicating the start time of a frame or a time-slot inaccordance with an adapted synchronization with respect to start time ofa TDMA frame or time-slot.
 15. A mobile station arranged forcommunicating on a direct mode channel divided into frames consisting oftime-slots, comprising:memory means; a transceiver arranged to receiveand transmit transmissions of said mobile station, a controller forcontrolling operation of said mobile station, a synchronizator forshifting the start time of a frame or a time-slot of a transmission onsaid direct mode channel with respect to the start time of a frame or atime-slot in accordance with the synchronization of a specific controlchannel of a mobile communications system in response to thesynchronization information transmitted by a second mobile station, or abase station or a repeater station of the mobile communications system,wherein:said memory means is arranged to store therein saidsynchronization information transmitted by said second mobile stationsaid base station or said repeater station of the mobile communicationssystem and received by said transceiver, said synchronizationinformation indicating the start time of a frame or a time-slot inaccordance with the adapted synchronization with respect to a start timeof a TDMA frame or time-slot.